Magnesium chloride is presently used extensively in the production of metallic magnesium by electrolysis of the fused chloride. Widely used sources of magnesium chloride are sea water, salt lakes and similar brines from which a magnesium chloride concentrate is recovered. When this concentrate is heated to drive off the physically combined water, a magnesium chloride hydrate is formed which contains a substantial amount of chemical combined water. It is essential, however, to the recovery of magnesium metal by electrolysis of fused magnesium chloride, that the magnesium chloride feed to the cell be substantially free of water of crystallization, for otherwise H.sub.2 will be formed at the cathode; and O.sub.2 and compounds of oxygen will be evolved at the anode thus causing rapid erosion of the anodes. Also, any MgO and MgOHCl in the feed must be removed otherwise particulate MgO will form and sink to the bottom of the cell to form a sludge which reduces the operating efficiency of the cell, requires much labor and equipment to remove from the cell, forms irreversible deposits which eventually prevent salt circulation and require cell shut-down and rebuilding, as well as reducing the recovery of metallic magnesium which becomes trapped in the cell "smut" deposits.
One present practice for forming a substantially dry magnesium chloride suitable as a feed material for electrolytic cells is to use a spray-dry unit including cyclone separator. Units of this type are traversed by heated gases having preferably an air inlet temperature of about 1000.degree. F. and an air outlet or discharge temperature of about 500.degree. F. The magnesium chloride concentrate is fed into the inlet of the spray drier and is recovered from the outlet in the form of substantially dry magnesium chloride of no more than about 3-5% water--which, after being further dehydrated in a melt cell, has been found to be sufficiently free of water to be used effectively in electrolytic cells to produce magnesium metal. However, this spray-dried product is a dry, very light, fluffy material, as large as 800 microns in size, thin-walled hollow spheres, fragile and readily fragmented into very thin particles of low bulk density. As the consequence, it is difficult to handle mechanically, store, or to control when feeding into melt cells for forming molten magnesium chloride.